Polyimides are widely used in the aerospace industry and electronics industry, because of their toughness, low density, thermal stability, radiation resistance and mechanical strength. However, it is recognized that polyimides are difficult to process. The processing problems arise from the insolubility of polyimides in most of the more common solvents. Consequently, products were fabricated from polyamide-acid intermediates, which are more soluble but less stable, and then imidized to provide the desired end product. The disadvantage of this process is that the water liberated during the imidization of the polyamide-acid forms undesirable voids or surface irregularities in the final product which reduce its mechanical properties.
Another approach is to provide a fully imidized prepolymer having reactive end groups. In this way, the water formed during imidization is removed before the final cure of the prepolymer. The resulting polyimide product is typically a thermoset plastic. However, the imidized prepolymers are not as soluble as would be desired.
It has been suggested that polyimides having a single hexafluoroisopropylidene linking group in the diamine or dianhydride comonomers have improved solubility properties. Several patents disclose polyimides prepared from diamines of this type. For example, U.S. Pat. No. 3,356,648 to Rogers discloses polyimides prepared from 2,2-bis(4-aminophenyl) hexafluoropropane and 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride; U.S. Pat. No. 3,959,350 to Rogers discloses polyimides prepared from 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane and other diamines; U.S. Pat. No. 4,592,925 to DuPont et al. discloses polyimides prepare from 2,2-bis(3-aminophenyl) hexafluoropropane; U.S. Pat. No. 4,111,906 to Jones et al. discloses polyimides prepared from 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane; and U.S. Pat. No. 4,477,648 to Jones et al. discloses polyimides prepared from 2,2-bis[(2-halo-4-aminophenoxy)phenyl] hexafluoropropane and a dianhydride including, inter alia, 2,2-bis(3,4-dicarhoxyphenyl)-hexafluoropropane dianhydride.